Quote possibly empty variable to avoid error message

On RedHat 6.4 workstation the module doesn't find the necessary system
information, and gives an error message when the last parameter to the
string function is not present.
Read /etc/system-release if others not parseable
Corrected mismatched if-else-endif arguments
Merge pull request #53 from rolk/053_distrib

Fix bug in reporting of system release (on RHEL 6.4)